The warming over the southeastern shelf is closely interwoven with the absence of sea ice, but it is not clear whether it is solely changes in atmospheric patterns that have resulted in a decrease in ice and in warming, or whether the ocean plays a role. We will explore four mechanisms in this section. First, changes in the winter winds directly affect both the temperature of the Bering Sea shelf and the formation and advection of sea ice, but it is not solely wind direction that influences ice formation. The origin of the air itself impacts ice formation, since frigid air temperatures are a necessary condition for the creation of sea ice (Stabeno et al., 2001; Bond and Adams, 2002; Rodionov et al., 2007). Second, shortening of the ice "season" through a later fall transition and/or an earlier spring transition can limit the southern extent of the ice. The third mechanism is a feedback effect: the presence of warmer shelf water during summer (due to a lack of ice the previous winter) results in warmer fall temperatures, which would delay the advection of ice by increased melting of the leading edge. Finally, changes in the flow through Unimak Pass during winter can contribute to increased wintertime ocean temperatures over the southeastern shelf, which also could delay the advection of ice.
These mechanisms interact in complex ways and are not independent of each other. Quantitative evaluation of these interactions is best accomplished using numerical ocean models. For the present study, however, we use a combination of discussion of earlier studies, simple models and a comparison between 2 years, 1975 and 2002, to help illustrate the effects of the mechanisms introduced above. The years 1975 and 2002 were chosen to assist in this exploration because they had similar winter weather conditions overall, but decidedly different ice extents and ocean temperatures the previous fall.
To prepare for the discussion in the following four sections, we begin by showing the mean atmospheric circulation for the two winters. The 700 hPa geopotential height anomaly maps for December–March of the winters of 1975 and 2002 (Fig. 11) both include lower than normal heights over the northern Bering Sea and higher than normal heights over the central North Pacific south of the Aleutians, implying in each case relatively strong eastward flow aloft. This anomalous westerly flow off the eastern tip of Siberia brought about lower tropospheric air temperature anomalies of approximately –3°C in 1975 and approximately –1°C in 2002 in an east–west band across the Bering Sea (not shown). While the overall weather pattern was similar for the two winters, it is instructive to examine two aspects of the air–sea interaction in greater detail. Daily time series of the net surface heat fluxes in the southern Bering Sea for 1975 and 2002 (Fig. 12A and B) show the character of the sub-seasonal fluctuations in the surface heat exchange. Through February, both years had substantial loss of heat from the ocean to the atmosphere. In 1975, however, there was one additional outbreak of cold arctic air in March.
Fig. 11. Mean 700 hPa geopotential height anomaly (m) for (top panel) December 1974–March 1975 and (bottom panel) for December 2001–March 2002. Anomalies are relative to a baseline period of 1968–1996.
Fig. 12. Daily values of the net surface heat flux (W m2) at (A) Site 2 (57°N, 164°W) and (B) at Unimak Pass (54°N, 165°W). Panels (C) and (D) show the along-peninsula wind stress (N m2) for the same locations for 1 November 1974–31 May 1975. Negative is toward the west-southwest. The dotted lines represent climatological means.
In both years, the maximum ice extent over the western portion of the shelf was similar, covering most of the slope and outer shelf. Over the eastern portion of the shelf, however, the ice cover was more extensive in 1975 than in 2002 (Fig. 2). Maximum ice extent occurred in February in both years (Figs. 2 and 3). Although the negative heat fluxes through February were larger in 2002, the maximum ice extent was greater in 1975. In addition, the cooling event in mid-March resulted in persistence of ice over the shelf in the 1975 compared with 2002 (Fig. 3).
There is a direct link between strong, arctic winds which blow southward and the formation of sea ice. These frigid winds are critical not only in the formation and advection of ice, but also in cooling of the water column before the arrival of ice. The atmospheric circulation patterns that result in extensive ice are complex (e.g., Rodionov et al., 2007), but some general statements can be made. Patterns that pump warm, maritime air northward into the Bering Sea and then southward over the eastern shelf do not produce extensive ice, while patterns in which cold, arctic air blows southward typically result in extensive ice formation (Stabeno et al., 2001; Bond and Adams, 2002). We discuss both the winter and spring atmospheric patterns here, as have been discussed in several recent publications.
The atmospheric circulation during the winters since 2000 has favored relatively low sea-level pressure (SLP) over the Bering Sea (Overland and Wang, 2005b). While the meridional component of the winds has been fairly typical in this period, there has been reduced cooling of the ocean by the atmosphere. The lower SLP over the Bering Sea signifies a greater proportion than usual of the warm and moist air masses of maritime origin that accompany cyclonic disturbances, and a reduced frequency of cold and dry air masses of continental or arctic origin that accompany high-pressure anticyclones (e.g., Stabeno et al., 2001; Bond and Adams, 2002).
Such a change in air temperature not only directly impacts the formation of sea ice, but it also influences the cooling of the water column. As can be seen in Fig. 5, the winter ocean temperatures at M2 from 2000 to 2005 have been several degrees warmer than ocean temperatures from 1996 to 2000. This has been true in December and January, even before ice arrived at the mooring. The lack of cold winds out of the north during late fall and early winter contributed to these warmer ocean temperatures. It is not clear, however, whether the water column has been warmer because of warmer fall winds, or because of the higher heat content in the water column from the previous summer.
A somewhat different mechanism has been operating recently during the spring. As shown by Overland and Wang (2005b), the atmospheric circulation in spring since 2000 has repeatedly featured low pressure over Siberia and high pressure over Alaska. The consequence of this has been northward wind anomalies over the Bering Sea and hence both relatively warm air and anomalous northward transport of sea ice. The impact of these anomalously northward winds has contributed to the early disappearance of ice over the southern shelf (Fig. 3) compared with the 1990s and earlier. The patterns of high and low-pressure anomalies are clearly related to large-scale atmospheric circulation (Overland and Wang, 2005b). As these patterns relax, it is expected that the Bering Sea will become cooler.
There are three primary mechanisms that can cool the water column over the southeastern Bering Sea shelf: direct heat loss to the atmosphere; the horizontal transport of cold water into the region; and the cooling of the water column through ice melt.
We have already discussed the first of these mechanisms: the similar rates of cooling early in the winter season in 2002 and 1975 (Fig. 12A and B). The summer ocean temperature during 2001 was particularly warm (Fig. 5), and this could have contributed to the ice extent in February 2002 being less than that in February 1975. In addition, 1975 had a period of strong cooling after mid-March. These arctic-air outbreaks are particularly effective at producing and advecting ice near the end of winter, when the water column is already near the freezing point.
While the transport of cold water southward could in theory be very effective in cooling the water column, it fails to do so for a couple of reasons. First, there is great variability in the direction and magnitude of currents (Fig. 9). Second, during the winter, the mean monthly transport at M2 is northward (Fig. 10A), so, on average there is northward flux of heat over the shelf during winter. Episodic advective events, however, do play a role in changing the temperature of the shelf. The sudden warming of the water column by more than 1°C when ice recedes from the mooring site (e.g., the sudden change from ~1.7°C indicated by black during several years) is evident in Fig. 5A. This is especially clear in January 2000, when the ocean temperature quickly warmed by >2°C.
The latent heat flux due to the melting of ice is probably one of the most effective ways of cooling the water column. Consider a simple box model with a 70-m-deep water column; melting a 1-m thickness of ice will cool that column by ~1.1°C. Ice is typically advected at ~2% of the wind speed, so during a day with wind speeds of V m s–1, ice would be pushed ~2 V km (if it did not melt). However, it does melt, and the warmer the water the more rapidly the ice melts. For example, in water 1.1°C above 0°C, the 1 m of ice would melt, so that the ice edge would advance only ~V km in a day. If the water column was 2.2°C above 0°C the ice edge would advance by 0.67 V km in a day. These are just simple estimates to show the impact of warmer water on the ice extent, since ice does not melt instantaneously when advected over warmer water. Warm water can delay the advance of sea ice, but the advance itself is still dependent upon the wind.
A delay in the fall atmospheric transition and/or an earlier spring atmospheric transition will result in a shorter ice season. During summer, winds over the southeastern Bering Sea are northward, introducing warmer air masses over the Bering Sea (Fig. 10C). For ice to form, the winds must first shift to southward. In four of the last 5 years, ice has arrived later than it did in the 1990s (Fig. 4), indicative of a later fall atmospheric transition. An earlier spring transition was discussed in Overland and Stabeno (2004). Cold temperatures in spring are more effective in creating the cold pool than are cold conditions in winter, since if ice retreats early strong storms can mix and advect water onto the shelf warming the cold pool (e.g., 1998, Fig. 5A). Using the examples of 1975 and 2002, recent winters have tended to be shorter (rather than milder) than those of the past. The degree to which these regional effects are related to changes in the seasonality of the large-scale atmospheric circulation is unknown. Certainly, an early transition in 2002 to spring-time conditions contributed to warmer, near-bottom temperatures at M2, when compared with conditions in 1975.
One possible interpretation of the lack of sea ice along the Alaska Peninsula since the mid-1990s is that warmer water is being advected northeastward along the peninsula (see Fig. 2; and Fig. 5 in Stabeno et al., 2002b). The flow through Unimak Pass is an important source of nutrients for the southeast shelf (Stabeno et al., 2002b) and also may represent a significant source of heat. Stabeno et al. (2002b) showed that the transport through Unimak Pass is strongly correlated with the local along-peninsula component of the wind. Southwestward winds confine the Alaska Coastal Current (ACC) along the south side of the Alaskan Peninsula. When so confined, much of the ACC will flow through Unimak Pass, the first (easternmost) broad pass in the Aleutian Arc that the ACC encounters (Nof and Im, 1985). Once through the pass, the flow bifurcates (Fig. 13) with part flowing northwestward along the 100-m isobath and a portion flowing northeastward along the Alaska Peninsula (Stabeno et al., 2002b). Unfortunately, there are limited observations of how strong this eastward flow along the peninsula is during the winter, and even during spring and summer it is only a few centimeters per second.
Fig. 13. Satellite-tracked drifter trajectories. The black "+" indicate the position of the drifter at the first of each month. The drifters each had "holey sock" drogues centered at ~40 m depth.
The time series of the along-peninsula component of the wind stress (Fig. 12C and D) indicate that, due to the winds, northward transports through Unimak Pass were probably larger in the early winter of 2002 than in 1975. Specifically, the mean along-peninsula wind stress was –0.04 N m–2 in November–December of 2001 and 0.03 N m–2 in November–December of 1974. If we consider just the period when M2 has been occupied (1995–2006), the overall warmth of the shelf in recent years (2001–2005) as compared to the earlier 6 years is consistent with the anomalous westward winds (due to a stronger than normal Aleutian Low) that have prevailed over the same period.
In addition, ocean temperatures in the Gulf of Alaska near Seward have increased over the last 30 years by ~1°C (T. Royer, personal comm.). The temperature along the south side of the Alaska Peninsula, as observed at Pavlof Bay mooring (Fig. 8), is warmer since 2001 when compared to the five previous years. The reason that Pavlof Bay temperatures mimic those at M2 is likely that both are affected by the large-scale atmospheric forcing, plus some "spillover" of the more regional forcing in the Bering Sea. For instance, if sea ice occurs along the north side of the Alaska Peninsula, then winds blowing from the north across the ice would be colder, cooling water along the south side of the Alaska Peninsula where the terrain is flat, as it is in the vicinity of Pavlof Bay.
It appears that warmer water is flowing through Unimak Pass, either as a result of general warming of the Gulf of Alaska or local lack of cooling along the south side of the peninsula (e.g., Pavlof Bay). These warmer temperatures together with enhanced northward transport through Unimak Pass could contribute to the observed warming over the southeastern Bering Sea shelf during either the fall or winter. If the eastward flow persists during winter, then it would introduce warmer water along the peninsula, thus limiting the advection of ice over the southeastern shelf and perhaps enhancing melting. If the flow persists during the summer and/or fall, the warmer water introduced during fall could also delay arrival of ice. Either way, this particular mechanism is probably limited to the region south of M2.
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